1. Field of the Invention
This invention relates generally to a process for the perbromination of phenol and diphenyl ether by brominating the appropriate compounds at a temperature in excess of 35.degree. C. in bromine as the sole reaction medium in the presence of selected catalysts.
2. Description of the Prior Art
In the past, numerous processes have been employed for replacing all the nuclear hydrogen atoms in aromatic compounds such as diphenyl ether and the like. None of these processes has been found to be entirely successful; all have been found to have disadvantages.
Prior art perbromination processes in general have involved use of an up to about 20% excess of bromine in the presence of various kinds of reaction media and solvents such as ethylene dibromide, carbon tetrachloride, chloroform, methylene bromide, acetylene tetrachloride, and the like depending on the particular aromatic compound to be brominated. Perbromination of aromatic compounds has also been carried out in reaction media such as oleum, concentrated sulfuric acid, fuming sulfuric acid, liquid sulfur dioxide, and the like.
Each of these process approaches has serious disadvantages for perbromination, especially for commercial operations. The use of halogenated organic solvents has disadvantages which include low productivities, undesirably slow reaction rates, and the necessity for recovery of the solvents for recycle. In some cases this technique results in the introduction of small but significant amounts of chlorine into the final product, thus limiting product quality. In addition a number of non-condensed aromatic compounds do not undergo perbromination satisfactorily under conditions which can readily be employed using halogenated organic solvents.
Disadvantages in the use of oleum as a reaction medium include, in addition to all the usual problems associated with its handling on a large scale, the required use of large volumes, thus reducing reactor productivity, difficulties in product isolation, disposal of large volumes of sulfuric acid, and poor quality products resulting from the presence of sulfur-containing contaminants such as brominated benzene-sulfonic acids. The latter contamination problem is also encountered in the use of liquid sulfur dioxide as reaction medium.
The inherent difficulties attendant on large scale perbromination are illustrated by several exotic synthetic chemical approaches proposed for hexabromobenzene. For example, the chemical literature describes the pyrolysis of octabromocyclohexenones and the reaction of hexabromocyclopentadiene with tribromoacetaldehyde at super-atmospheric pressures and highly elevated temperatures.
Another approach to the preparation of hexabromobenzene and other perbrominated aromatic compounds has been the use of bromine/chlorine mixtures as the bromination agent. This process suffers the serious disadvantage of introducing small but significant amounts of chlorine into the final products thus limiting their final quality.
Yet another process which has been described for the polybromination of aromatic compounds requires the use of powerful mechanical mixers to maintain partially brominated solid intermediates in a sufficiently fine state of subdivision to permit access of bromine liquid or vapor in attempts to achieve high levels of bromination.
It has also been proposed to prepare completely brominated derivatives of aromatic compounds containing one or more phenyl groups through the use of bromine solvent (100% excess or more) and a bromine transfer catalyst at relatively low reaction temperatures of from about 10.degree. C. to ambient (i.e., about 20.degree.-25.degree. C.) A related process has involved ring bromination of aromatic compounds employing a minimum excess of 20% of bromine in the presence of halogenation catalysts at ambient (20.degree.-25.degree. C.) temperature. While these processes have, when carried to completion, permitted perbrominated product to be obtained in good yield, such results have been achieved only through acceptance of economic penalties. Inefficiencies introduced as a result of low reactor bromination temperatures make use of such processes on a commercial scale even less desirable.
Accordingly, it is an object of this invention to prepare pentabromophenol and decabromodiphenyl ether in high yield and purity by a relatively simple reaction using excess bromine as both reactant and the sole reaction medium.
Another object of the invention is to prepare perbrominated compounds of the character described substantially free from lower brominated products.
It is also an object to employ at least a 75% excess above the stoichiometric amount of bromine for the perbromination.
A further object is to carry out perbromination reactions of the character described at elevated temperatures in order to enhance significantly equipment productivities per unit volume and time without affecting product yield and qualities.